Sheep blowflies have evolved from a very successful class of carrion-breeding flies. As carrion breeders they fulfil an important function, accelerating the breakdown of carcases and the return of nutrients to the environment.

However, carrion is a suitable food source for many other animals and insects, as well as the many species of blowfly. Thus, a suitable carcase becomes a site of intense competition and less vigorous species are destroyed or pushed out.

The wool of a sheep with active fleece rot or a soiled breech harbours seething masses of bacteria. This provides an environment somewhat similar to that found in a fairly fresh carcase. Some of the more opportunistic flies have been able to make the transition from breeding on carcases to developing on suitable live sheep. These flies cost the Australian sheep industry well over $100 million per year in deaths, lost production and treatment costs.

The sheep blowflies show differing degrees of adaptation to breeding on live sheep. However, all prefer to lay on carrion if available. A large scale survey in the late 1970s, in which flies were bred from over 1400 strikes around Victoria, found major differences in the species of flies responsible. The appearance, importance and biology of the different flies are described below.

Lucilia cuprina

Primary green blowfly or Australian sheep blowfly

This small shiny green fly is "public enemy number one" as far as the Australian woolgrower is concerned. It was found in 96% of all strikes around Victoria, and was the only species present in over half of these. Not only does it initiate most strikes, it is the main species involved in their continuation.

Eggs are laid on areas of soiled wool on living sheep or openings in fresh carcases. Provided they remain moist the eggs take from eight to 36 hours, depending on temperature, to hatch. Drying is fatal to the eggs.

On carcases, competition from other fly species and predation by beetles overwhelms most of the maggots of Lucilia cuprina and few survive.

On live sheep the larvae (maggots) move down to the skin after hatching. Lucilia maggots are smooth and pale.

Young maggots cannot damage sound, healthy skin and are killed by drying. However, skin affected with fleece rot or scalded with urine or diarrhoea will "weep" fluid, and this allows the maggots to become established. Older maggots can attack and damage surrounding areas of healthy skin, thus allowing further eggs and maggots to establish readily.

After feeding for three or four days maggots drop from the sheep, generally at night. They therefore tend to be concentrated around sheep camps. Maggots burrow into the soil to pupate, becoming encased in a brown, barrel shaped puparium. In warm conditions, pupation occurs 2-3 days after the maggots leave the sheep. In late autumn, however, maggots do not pupate until the following spring; the fly overwinters as a maggot.

There is, however, a high death rate among overwintering maggots, so that the species must breed up each spring and summer from a small surviving population.

The pupal stage lasts for from six days in warm weather to 25 days when conditions are cool.

After emerging from the puparium, adult females require a protein feed before their eggs can develop. Protein may be obtained from carrion, struck sheep or, in some cases, manure. It takes at least 3-4 days before the female is ready to lay eggs. The size of the egg batch depends on the size of the female. Large females are those that were well fed as maggots. Under the most favourable conditions a generation of L. cuprina may be completed in about 17 days. In the field each female can lay two or three batches of 200 or more eggs.

Although exceptions occur, most L. cuprina only travel about 1-2 km in their lifetime: thus many attack sheep on the property on which they were bred.

Lucilia sericata

European green blowfly

This fly, which is virtually identical in appearance to Lucilia cuprina, is the main sheep blowfly in the United Kingdom. However, it is of negligible importance to Australia. It was found in less than 3% of Victorian strikes, and never as the sole species.

Calliphora stygia

Common brown blowfly

Calliphora stygia is a large native Australian blowfly with a grey thorax and yellow-brown mottled abdomen.

Apart from being a common nuisance in houses, it is also of considerable importance in flystrike. Like the Lucilias and other Calliphoras it is a "primary" fly, capable of initiating flystrike on a previously unstruck sheep.

C. stygia occurred in 27% of the strikes studied throughout Victoria, although rarely as the sole species. It is adapted to cooler conditions than other flies, being found as an adult throughout winter in southern Victoria. This adaptation to the cold gives it an advantage on carrion during the cooler months, and in spring in particular many thousands of these flies can develop from carcases. The maggots resemble those of Lucilia but are larger.

In summer, high temperatures and competition from species such as Chrysomya rufifacies prove fatal, and C. stygia becomes scarce. In Western Australia, C. stygia is displaced by the very similar Calliphora albifrontalis.

Calliphora augur

Lesser brown blowfly (eastern)

This species is generally slightly smaller than C. stygia, which it otherwise resembles except for a dark blue patch on the abdomen.

It occurred in 23% of all strikes studied throughout Victoria, and in over one-third of strikes in the south-west. Like C. stygia it is commonly found in houses, but unlike most other blowfly species it lays living maggots, not eggs. This gives it a head start on other species, enabling it to exploit small carcases as well as other carrion. Maggots resemble those of C. stygia.

Calliphora nociva

Lesser brown blowfly (western)

This species closely resembles C. augur except for the colour patch on the abdomen, which is a much brighter blue on C. nociva than on C. augur. It displaces C. augur in Western Australia.

Although it also occurs throughout most of Victoria, it is more abundant in the west of the state, although always less so than C. augur. C. nociva was involved in 7% of the strikes studied in the western regions of Victoria but only 1% of eastern strikes.

Chrysomya rufifacies

Hairy maggot fly

This green native fly is stouter than L. cuprina and has dark bands on the abdomen. It requires higher temperatures than the other Victorian sheep blowflies. It is a "secondary" blowfly which does not initiate strikes. It is adapted to breeding on carrion, although it will sometimes attack already-struck sheep.

The rough, dark maggots tend to burrow into the flesh, so it can greatly increase the severity of pre-existing strikes.

Chrysomya rufifacies was found in 8% of strikes throughout Victoria overall, but was most common in the north. This fly was common at Hamilton, but usually ignored struck sheep, preferring to breed on carrion.

The maggots of Chrysomya rufifacies attack and devour other maggot species, especially on carrion, and during summer they contribute to the rapid decline in the numbers of the brown blowflies.

Chrysomya varipes

Small green blowfly

This fly is a miniature of Chrysomya rufifacies and is of similar habits. It is rarely involved in flystrike in Victoria.

Australophyra rostrata

This small dark blue fly is primarily a carrion breeder. It is late into strikes and carrion. It is considered to be a "tertiary" fly, living in the exudate left as an aftermath of flystrike and doing little damage. It occurred in 7% of strikes around Victoria.

Recommendations for prevention

Any flystrike prevention program must be aimed mainly at Lucilia cuprina. As this fly breeds almost exclusively on susceptible living sheep, prevention of flystrike will reduce the numbers of this fly.

This in turn reduces the pressure on any sheep that do become susceptible to flystrike, thus helping sheep to help themselves.

Small flystrikes early in the season provide the means whereby numbers of Lucilia cuprina build up from the relatively few maggots that survive the winter. These can be greatly reduced by:

Correct tail docking

Mulesing of sheep kept for wool. Surgical mulesing will cease in 2010. Non-surgical alternatives are under development.

Prevention of scouring by a good worm control program

Breeding breech-strike resistant sheep

Selection away from harsh-woolled, wrinkly sheep

Finally, destruction of flystruck crutchings kills many maggots that would otherwise survive. Jetting should not be used as a substitute for other management practices, but can be a valuable aid.

Acknowledgements

Photographs used in this Information Note are by Colin Brimblecombe. The previous version was published in December 1998.

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